An apparatus for detecting a surface flaw of a pipe with the use of electromagnetic induction is publicly known. For example, an apparatus for detecting an outer surface flaw of a pipe with the use of electromagnetic induction is disclosed in Japanese Patent Provisional Publication No. 60-11,157 dated Jan. 21, 1985, which comprises: at least one cylindrical primary coil, a high frequency electric current generator, a plurality of probe coils, i.e., a plurality of cylindrical secondary coils, a multiplexer and a signal processing circuit (hereinafter referred to as the "prior art").
The at least one primary coil surrounds a pipe to be inspected, and is coaxial with the pipe. In other words, the pipe is coaxially inserted into the at least one primary coil. The inner peripheral surface of the at least one primary coil is spaced apart from the outer peripheral surface of the pipe by a prescribed distance.
The high frequency electric current generator supplies high frequency electric current to the at least one primary coil to cause the at least one primary coil to produce an AC magnetic field, and the magnetic flux density of the AC magnetic field varies in response to an outer surface flaw of the pipe.
The plurality of secondary coils are arranged along the outer surface of the pipe at prescribed intervals in the circumferential direction of the pipe in the close vicinity of the at least one primary coil. The axis of each of the plurality of secondary coils is arranged at right angles to the axis of the at least one primary coil. Each of the plurality of secondary coils produces an AC voltage proportional to the density of a component parallel to the axial direction of each of the plurality of secondary coils, of the magnetic flux interlinking with each of the plurality of secondary coils, of the AC magnetic field of the at least one primary coil. The plurality of secondary coils constitute, together with the at least one primary coil, a detecting probe, and the detecting probe is moved relative to the pipe in the axial direction of the coil.
The multiplexer repeatedly takes out the AC voltage signals from the plurality of secondary coils sequentially in the order of arrangement of the plurality of secondary coils at a prescribed sampling cycle period T.
The signal processing circuit comprises a synchronous detector, a delay circuit and an adder.
The synchronous detector sequentially and synchronously detects the AC voltage signals from the plurality of secondary coils, taken out by the multiplexer, with the high frequency electric current from the high frequency electric current generator as the reference signal, thereby eliminating noise signals from the AC voltage signals from the plurality of secondary coils, and at the same time, converting the AC voltage signals into DC voltage signals. Each value of the thus converted DC voltage signals is proportional to the depth of an outer surface flaw of the pipe.
The delay circuit causes delay of the DC voltage signals from the synchronous detector by a period of time equal to the above-mentioned sampling cycle period T.
The adder adds the thus delayed DC voltage signal from the delay circuit to a DC voltage signal from the synchronous detector in the next sampling cycle period for each of the plurality of secondary coils, thereby obtaining a DC voltage signal with a minimized detection error in the pipe axial direction of the outer surface flaw of the pipe for each of the plurality of secondary coils.
In the above-mentioned prior art, it is possible to detect the presence and the depth of the outer surface flaw of the pipe with a minimized detection error in the axial direction of the pipe, by sequentially detecting a differential voltage signal proportional to the depth of the outer surface flaw of the pipe, which is obtained by subtracting the bias voltage signal of each of the secondary coils resulting from an inclination or other condition of each of the secondary coils, on the one hand, from the DC voltage signal from the adder for each of the secondary coils, on the other hand.
According to the prior art, it is possible to detect an outer surface flaw of the pipe without overlooking any other surface flaw in the pipe axial direction, even when carrying out detecting operation of the outer surface flaw of the pipe while moving, relative to the pipe, the detecting probe comprising the at least one primary coil and the plurality of secondary coils at a high speed in the axial direction of the pipe.
The above-mentioned prior art, which relates to the detection of an outer surface flaw of a pipe, is also applicable to the detection of an inner surface flaw of each pipe constituting a pipeline, by causing the detecting probe comprising the at least one primary coil and the plurality of secondary coils to travel through the pipeline. However, when detecting any of the outer surface flaw or the inner surface flaw of the pipe, the prior art has the following drawbacks.
More specifically, the magnetic flux of the AC magnetic field of the at least one primary coil, which is distributed in the axial direction of the pipe in the space near the outer surface or the inner surface of the pipe, comes into an outer or inner surface flaw of the pipe, if any, and as a result, the magnetic flux density in the space near the pipe portion containing the outer or inner surface flaw shows a normal distribution having a peak of the lowest density at the position of the flaw center. This means that, the magnetic flux has the lowest density at the position of the flaw center, and consists only of a component parallel to the axial direction of the at least one primary coil. On the other hand, the magnetic flux has the highest density at the position distant from the flaw center, and consists only of a component parallel to the axial direction of the at least one primary coil. In the middle between the position of the flaw center and the position distant from the flaw center, the magnetic flux density becomes higher as the distance from the position of the flaw center increases. The magnetic flux is analyzed into a component parallel to the axial direction of the at least one primary coil and a component at right angles to the axial direction of the at least one primary coil, and the latter component increases as the distance from the position of the flaw center increases to reach the maximum, and then decreases. Therefore, in a space near the pipe portion containing an outer or inner surface flaw, the highest density of the component of the magnetic flux, which component is at right angles to the axial direction of the at least one primary coil, exists in the middle between the position of the flaw center and the position distant from the flaw center.
The difference in the magnetic flux density between the lowest density at the position of the flaw center and the highest density at the position distant from the flaw center corresponds to the depth of the flaw. The highest density of the component of the magnetic flux, which component is at right angles to the axial direction of the at least one primary coil, at a position between the position of the flaw center and the position distant from the flaw center also corresponds to the depth of the flaw. Since, in the above-mentioned prior art, the plurality of secondary coils are arranged so that the axis of each of the plurality of secondary coils is at right angles to the axis of the at least one primary coil, each of the plurality of secondary coils senses a component at right angles to the axial direction of the at least one primary coil, i.e., a component parallel to the axial direction of each of the plurality of secondary coils, of the magnetic flux of the AC magnetic field of the at least one primary coil, which magnetic flux interlinks with each of the plurality of secondary coils, and produces an AC voltage proportional to the density of the above-mentioned component. Therefore, it is possible to detect the depth of the outer surface flaw or the inner surface flaw of the pipe, by processing the AC voltage signal produced by each of the plurality of secondary coils.
However, when a first flaw, a second flaw and a third flaw each having a respective depth are present in this order on the outer or inner surface of the pipe at close intervals in the axial direction of the pipe, the density of the magnetic flux in the axial direction of the pipe, of the AC magnetic field of the at least one primary coil, in the space near the pipe portion containing these flaws, shows a distribution in which three normal distributions of the magnetic flux density corresponding respectively to these three flaws partly overlap in the axial direction of the pipe. In such a distribution of the magnetic flux density, a distribution of the magnetic flux density at a position between the center position of the first flaw and a position opposite to the second flaw relative to the first flaw, and a distribution of the magnetic flux density at a position between the center position of the third flaw and a position opposite to the second flaw relative to the third flaw, are not affected by the distribution of the magnetic flux density corresponding to the second flaw. Therefore, the highest densities of the components at right angles to the axial direction of the at least one primary coil of the magnetic flux in these two intermediate positions correspond respectively to the depth of the first flaw and the depth of the third flaw. On the contrary, a distribution of the magnetic flux density at a position between the center position of the first flaw and the center position of the second flaw, and a distribution of the magnetic flux density at a position between the center position of the second flaw and the center position of the third flaw, are affected by the distributions of the magnetic flux density corresponding respectively to the first flaw and the third flaw. Therefore, the highest densities of the components at right angles to the axial direction of the at least one primary coil of the magnetic flux in these two intermediate positions do not accurately correspond to the depth of the second flaw. Thus, the depth of the second flaw cannot be accurately detected by the prior art.
Also when four or more flaws are present on the outer or inner surface of the pipe at close intervals in the axial direction of the pipe, the same problem as described above is posed for the flaws other than those at the both ends.
Under such circumstances, there is a strong demand for the development of an apparatus for detecting, with the use of electromagnetic induction, an inner surface flaw of each pipe constituting a pipeline, which, when detecting an inner surface flaw of each of a plurality of pipes forming the pipeline, permits accurate detection of the depth of each of three or more inner surface flaws of the pipe even when these inner surface flaws exist on the inner surface of the pipe at close intervals in the axial direction of the pipe, but an apparatus provided with such properties has not as yet been proposed.